1. Field of the Invention
The present invention relates to an image processor, and more particularly to an image processor in which half-toning is performed when image data of continuous-tone are inputted so that an image attended with assigning tone levels may be printed and recorded.
2. Description of the Related Art
A dither method, a gray level pattern method and the like have been heretofore known as techniques for performing intermediate tone expression. These techniques have such a problem that a moire is generated in a reproduced image in case the image is a mesh point image. Further disadvantage is that when the image includes a line, character and the like, an edge of the line or character becomes broken into pieces.
Thus, an error diffusion method is used sometimes as a technique for solving those problems. In the error diffusion method, however, there is also such a problem that when a highlighted portion is processed, recorded dots are concentrated at undesired areas and fiber-like noises appear conspicuously. In order to solve these problems, it is proposed to use an image processor having means for selectively using one of the dither method and the error diffusion method depending on the gray level of the input data, as disclosed in JP-A-2-31562.
In a conventional image processor, however, the dither method and the error diffusion method are simply changed from one to the other. Therefore, in case of an image having a gradation of the tone level, which changes continuously, the image quality of the printed image changes at a position where the dither method and the error diffusion method are changed over, resulting in an ugly appearance.